On the Location of Stain in Mitochondria
The degree of matrical staining in isolated and tissue mitochondria has been shown to be a function of their state of oxidative-phosphorylation at the time of fixation (J. Histochem. Cytochem. 15:752, 1967). High resolution EM in conjunction with these studies revealed amplitude contrast images 10 AU and larger in diameter which are independent of the focal plane. These amplitude densities have been interpreted as individual aggregates of heavy metal stain (J. Cell Biol. 35:55A, 1967). Two approaches have been used in an effort to locate such stain aggregates.(1) Four micron epoxy sections of skeletal muscle, cardiac muscle, and liver were stained with uranyl acetate, lead citrate, or both, following the methods used to stain 500 AU sections for EM. After staining, the thick epon sections were re-embedded and thin sections cut perpendicular to the stained surface. Either or both of these metal stains penetrate 500 to 1000 AU into the thick section and give the tissue increased contrast similar to that due to staining after thin sectioning. The mitochondrial matrices are lightly stained indicating that they are not in a state of active oxidative-phosphorylation. Figure 1 shows such mitochondria at low magnification before (Figure 1a) and after (Figure lb) restaining with uranyl acetate and lead citrate. After restaining it is no longer possible to identify the original stained surface.